Masculinized finger length patterns in human males and females with congenital adrenal hyperplasia.

The ratio of the length of the second digit (2D) to the length of the fourth digit (4D) is greater in women than in men. Since androgens are involved in most somatic sex differences and since the sexual dimorphism in 2D:4D is stable from 2 years of age in humans, it was hypothesized that finger length pattern development might be affected by early androgen exposure. Human females with congenital adrenal hyperplasia (CAH) are exposed prenatally to higher than normal levels of adrenal androgens, providing an opportunity to test the effects of early androgen exposure on digit ratios. The 2D:4D was calculated for females with CAH, females without CAH, males with CAH, and males without CAH. Females with CAH had a significantly smaller 2D:4D on the right hand than did females without CAH. Males with CAH had a significantly smaller 2D:4D on the left hand than did males without CAH. A subset of six males with CAH had a significantly smaller 2D:4D on both hands compared with their male relatives without CAH. These results are consistent with the idea that prenatal androgen exposure reduces the 2D:4D and plays a role in the establishment of the sex difference in human finger length patterns. Finger lengths may therefore offer a retrospective marker of perinatal androgen exposure in humans.

Sexual dimorphism in digit-length ratios of laboratory mice.

The lengths of the index finger (2D) and ring finger (4D) are sexually dimorphic in humans, and men have a smaller 2D:4D ratio compared to women. Prenatal androgens appear to be important in the development of the 2D:4D sex difference, since it has been reported in children as young as 2 years old, and since humans exposed to supernormal prenatal androgen levels display a smaller 2D:4D ratio. We tested whether another mammalian species in which the process of peripheral sexual differentiation is androgen-dependent might also show a sex difference in digit ratios. The 2D:4D ratio of adult outbred mice was calculated for both the left and right rear paws. A sex difference was observed in the right rear paw: female mice had a larger 2D:4D ratio than did males. We also found this difference in prepubescent weanling mice. This sex difference is in the same direction as that observed in humans, and suggests that sexual dimorphism in digit length ratios is a feature common to many, if not all, mammals. The mouse may therefore be a useful animal model for studying the factors that influence finger length patterns, which have recently been correlated with several specific behaviors and disease predispositions in humans.

Differences in finger length ratios between self-identified "butch" and "femme" lesbians.

There is indirect evidence that heightened exposure to early androgen may increase the probability that a girl will develop a homosexual orientation in adulthood. One such putative marker of early androgen exposure is the ratio of the length of the index finger (2D) to the ring finger (4D), which is smaller in male humans than in females, and is smaller in lesbians than in heterosexual women. Yet there is also evidence that women may have different sexual orientations at different times in their lives, which suggests that other influences on female sexual orientation, presumably social, are at work as well. We surveyed individuals from a gay pride street fair and found that lesbians who identified themselves as "butch" had a significantly smaller 2D:4D than did those who identified themselves as "femme." We conclude that increased early androgen exposure plays a role in only some cases of female homosexuality, and that the sexual orientation of "femme" lesbians is unlikely to have been influenced by early androgens.